The present invention relates to a gradient magnetic field measurement method and an MRI (magnetic resonance imaging) apparatus, and more particularly to a gradient magnetic field measurement method and an MRI apparatus that can accurately measure a gradient magnetic field actually applied.
FIG. 1 is a diagram for explaining a gradient magnetic field measurement pulse sequence for use in a gradient magnetic field measurement method disclosed in an article entitled xe2x80x9cNovel k-space Trajectory Measurement Techniquexe2x80x9d by Y. Zhang et al., in Magnetic Resonance in Medicine, 39: 999-1004 (1998).
The gradient magnetic field measurement pulse sequence J applies an excitation RF pulse R and a slice selective pulse Gs, applies a rephasing pulse Gr, and collects data S(1)-S(T) from an FID signal while applying an encoding pulse Ge having a spiral gradient waveform.
Next, data D(1)-D(Txe2x88x921), which have a phase difference xcex94xcfx86 as an angle, are obtained from the collected data S(1)-S(T). In particular, the following calculation is performed:
D(t)=S(t)xc2x7S(t+1)*,
wherein S(t+1)* represents the conjugate complex of S(t+1).
Then, gradient magnetic field differences xcex94G(1)-xcex94G(Txe2x88x921) are obtained from the data D(1)-D(Txe2x88x921) having a phase difference xcex94xcfx86 as an angle. In particular, the following calculation is performed:             Δ      ⁢              xe2x80x83            ⁢              G        ⁡                  (          t          )                      =                  arctan        ⁢                  xe2x80x83                ⁢                  {                      D            ⁡                          (              t              )                                }                            2        ⁢                  π          ·          γ          ·          z          ·          Δ                ⁢                  xe2x80x83                ⁢        t              ,
wherein arctan{} is the arc tangent function, xcex3 is the gyromagnetic ratio, z is the slice position on the gradient axis, and xcex94t is the time difference between the data S(t) and S(t+1).
Next, the gradient magnetic field differences xcex94G(1)-AG(Txe2x88x921) are integrated to obtain the gradient magnetic field G(1)-G(Txe2x88x921). In particular, the following calculation is performed:       G    ⁡          (      τ      )        =            ∑              t        =        1            τ        ⁢          Δ      ⁢              xe2x80x83            ⁢                        G          ⁡                      (            t            )                          .            
The result of the gradient magnetic field measurement is used for correcting the encoding pulse Ge. Moreover, it is used for analyzing eddy current or remanence.
Ideally, the result of the gradient magnetic field measurement with respect to the encoding pulse Ge shown in FIG. 1 would be such as shown in FIG. 2.
However, such a neat result as shown in FIG. 2 is not obtained in practice. Particularly, randomness will occur in a latter portion indicated by a broken line in FIG. 2. This is because a larger encoding pulse Ge increases the difference of gradient magnetic field strength within a sample, resulting in an observed FID signal reduced due to a phase shift generated within the sample. FIG. 3 shows the temporal variation of an FID signal. Basically, the FID signal exponentially decreases with time, but a lot of further smaller minimum portions appear because of the phase shift generated within the sample.
It is an object of the present invention to provide a gradient magnetic field measurement method and an MRI apparatus that can accurately measure a gradient magnetic field actually applied.
In accordance with a first aspect of the invention, there is provided a gradient magnetic field measurement method comprising the steps of: applying an excitation RF pulse, applying a pre-encoding pulse Pk, collecting data S(k, 1)-S(k, T) from an FID signal while applying an encoding pulse Ge having a gradient waveform to be measured, and repeating these steps K times with the magnitude of the pre-encoding pulse Pk varied; obtaining data D(1, 1)-D(1, Txe2x88x921), D(2, 1)-D(2, Txe2x88x921), . . . , D(K, 1)-D(K, Txe2x88x921) having a phase difference xcex94xcfx86 as an angle from the collected data S(1, 1)-S(1, T), S(2, 1)-S(2, T), . . . , S(K, 1)-S(K, T); adding data having corresponding magnitudes of the encoding pulse Ge to obtain added data d(1)-d(Txe2x88x921); obtaining gradient magnetic field differences xcex94G(1)-xcex94G(Txe2x88x921) from the added data d(1)-d(Txe2x88x921); and integrating the gradient magnetic field differences xcex94G(1)-xcex94G(Txe2x88x921) to obtain a gradient magnetic field G(1)-G(Txe2x88x921).
According to the gradient magnetic field measurement method of the first aspect, because the pre-encoding pulse Pk is varied in the collected data S(1, t), . . . , S(K, t), the magnitudes of phase are different. However, if the collected data are converted into data D(1, t), . . . , D(K, t) having a phase difference xcex94xcfx86 as an angle, the data come to have corresponding magnitudes of the encoding pulse Ge. On the other hand, because the pre-encoding pulse Pk is varied, the magnitude of phase shift within a sample is varied and portions at which an FID signal is reduced due to the phase shift are different among the data S(1, t), . . . , S(K, t). That is, although an FID signal observed in a certain portion in certain data is small, it is not small in the corresponding portion in other data. Then, adding these data gives d(1)-d(Txe2x88x921). Since a gradient magnetic field G(1)-G(Txe2x88x921) is obtained based on such added data d(1)-d(Txe2x88x921), the gradient magnetic field can be accurately measured.
In accordance with a second aspect of the invention, there is provided an MRI apparatus comprising RF pulse transmitting means, gradient pulse applying means, NMR signal receiving means and data processing means, wherein the RF pulse transmitting means applies an excitation RF pulse, the gradient pulse applying means applies a pre-encoding pulse Pk followed by an encoding pulse Ge having a gradient waveform to be measured, the NMR signal receiving means receives an FID signal while applying the encoding pulse Ge to collect data S(k, 1)-S(k, T), and, from the data S(1, 1)-S(1, T), S(2, 1)-S(2, T), . . . , S(K, 1)-S(K, T) collected by repeating the above operation K times with the magnitude of the pre-encoding pulse Pk varied, the data processing means obtains data D(1, 1)-D(1, Txe2x88x921), D(2, 1)-D(2, Txe2x88x921), . . . , D(K, 1)-D(K, Txe2x88x921) having a phase difference xcex94xcfx86 as an angle, adds data having corresponding magnitudes of the encoding pulse Ge to obtain added data d(1)-d(Txe2x88x921), obtains gradient magnetic field differences xcex94G(1)-xcex94G(Txe2x88x921) from the added data d(1)-d(Txe2x88x921), and integrates the gradient magnetic field differences xcex94G(1)-xcex94G(Txe2x88x921) to obtain a gradient magnetic field G(1)-G(Txe2x88x921).
The MRI apparatus of the second aspect is capable of suitably implementing the gradient magnetic field measurement method as described regarding the first aspect.
In accordance with a third aspect of the invention, there is provided the gradient magnetic field measurement method as described regarding the first aspect, comprising the steps of: defining time points Jk (k=1, . . . , K) dispersedly within a period of the encoding pulse Ge having a gradient waveform to be measured; and determining a magnitude of the pre-encoding pulse Pk so as to cancel an integral value of the encoding pulse Ge from its start time point to a time point Jk.
Checking on the time points at which an observed FID signal is reduced due to a phase shift within a sample, it is found that the time points are not concentrated at one location but are distributed over a plurality of locations.
The gradient magnetic field measurement method of the third aspect therefore defines a plurality of time points Jk (k=1, . . . , K) distributed within a period of the encoding pulse Ge, and determines the magnitude of the pre-encoding pulse Pk so as to eliminate the phase shift at each time point Jk. Thus, time points at which the FID signal is reduced due to a phase shift within the sample are differentiated among the pre-encoding pulses Pk, and therefore the gradient magnetic field can be accurately measured from the added data.
In accordance with a fourth aspect of the invention, there is provided the gradient magnetic field measurement method as described regarding the third aspect, comprising the steps of: applying an excitation RF pulse, but not applying a pre-encoding pulse Pk; collecting data S(1)-S(T) from an FID signal while applying an encoding pulse Ge having a gradient waveform to be measured; obtaining a temporal variation of the FID signal intensity from the collected data S(1)-S(T); and defining time points at which the FID signal intensity is minimum as the time points Jk (k=1, . . . , K).
The gradient magnetic field measurement method of the fourth aspect searches for time points at each of which a portion of the observed FID signal appears that is reduced due to a phase shift generated within the sample, and determines the magnitude of the pre-encoding pulse Pk so as to eliminate the phase shift at these time points. Thus, since the FID signal can be increased at a portion at which the FID signal would be reduced due to a phase shift without applying a pre-encoding pulse Pk, the gradient magnetic field can be accurately measured.
Accordingly, the gradient magnetic field measurement method and MRI apparatus of the present invention provide data compensated for reduction in the FID signal due to phase shifts generated within the sample, i.e., data with a generally good SNR altogether, and can therefore measure a gradient magnetic field accurately.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.